The present invention is in the field of automatic optical inspection techniques and relates to a method and an apparatus for inspecting articles utilizing a variable angle design.
The manufacture of various articles, such as integrated circuits, printed circuit boards, photolithographic masks, etc., requires them to be automatically inspected during progress on a production line. The timely detection of anomalies on the surface of such article is a very important factor subsequently leading to an increase in production yields.
Semiconductor wafers are inspected prior to and after every patterning procedure. Optical inspection systems typically employ such main constructional parts as illumination optics and collection-detection optics for, respectively, directing incident light from a light source onto the wafer to be inspected, and collecting light returned (scattered, reflected) from the wafer and directing the collected light onto a sensing means.
Prior-to-patterning inspection of wafers relies on the fact that light is scattered mainly from anomalies present on the generally flat and smooth surface of the non-patterned wafer. Thus, any detection of scattered light may be indicative of a defect.
However, when inspecting patterned wafers, scattered light can be caused by the pattern. Therefore, the detection of scattered light is not necessarily indicative of a defect. In order to detect defects on a patterned surface, templates formed by signals representative of detected light components scattered from periodic features of the pattern (i.e. dies of the wafer) are typically constructed and compared. Differences between the signals are indicative of light scattered from anomalies present on the surface of the article, and are therefore detected as defects. Another technique, so-called xe2x80x9cdie-to-diexe2x80x9d inspection, consists of comparing light scattered from an individual die to that of its xe2x80x9cneighborxe2x80x9d. Any detected difference in light components scattered from these two dies, is indicative of the absence or addition of some features in one of the dies as compared to the other, and is therefore considered to be a defect.
To facilitate meaningfull signal comparison, i.e. to successfully analyze the signals associated with different dies and cells of the wafer, it is desirable for the light collection system to collect light at one constant collection angle. The advantages of a single constant collection angle based technique are disclosed, for example, in U.S. Pat. Nos. 4,898,471 and 5,604,585, both of which relate to systems for detecting particles on the surface of a patterned article.
There is a need in the art to improve the conventional inspection techniques by providing a novel method and apparatus for optical inspection of articles utilizing a variable angle design.
It is a major feature of the present invention to provide such an apparatus that enables the signal-to-noise ratio of the detected signal to be significantly increased.
The main idea of the present invention is based on the following. An article under inspection is scanned region-by-region, and light scattered from each of the scan regions is collected with a certain maximum collection angle constant for each scan region. The maximum collection angle is a solid angle having a certain value and a certain central direction defined by a light collecting optics. The dimensions of a first collecting surface of the light collecting optics defines the value of the maximum collection angle, while the position of this surface defines the certain constant central direction of propagation of the collected light. A filter is selectively operable in the optical path of the collected light for selectively separating therefrom at least one light component propagating with a predetermined solid angle segment of the maximum collection angle. If any unexpected difference (defect) is detected at a specific location or, on the contrary, a probability of a defect exists at a specific location on the article (according to previous knowledge of the pattern structure of the article, which is typically the case), this location is inspected by varying (reducing) the collection angle. At this specific location, namely for this specific scan region, one or more light components of the collected light could be captured and directed on a detector, thereby increasing the signal-to-noise ratio of the detected signal, considering xe2x80x9cnoisexe2x80x9d as any light component coming from locations other than this specific location. These selected light components propagate with spatially separated solid angle segments of the maximum collection angle.
There is thus provided according to one aspect of the present invention an apparatus for optical inspection of an article, comprising an illumination unit generating an incident radiation and illuminating a predetermined region on the article, and at least one detection unit, wherein said at least one detection unit comprises:
(a) a light collecting optics that collects light scattered from the illuminated region with a predetermined constant maximum collection angle;
(b) a filter selectively operable in the optical path of the collected light for selectively separating therefrom at least one light component propagating with a predetermined solid angle segment of the maximum collection angle; and
(c) detector having a sensing surface for receiving collected light and generating data representative thereof.
The illumination unit may be oriented so as to illuminate the article either normally or at a grazing angle. The at least one detection unit operates in a perspective dark field imaging mode.
The filter may comprise a mask assembly. The mask assembly may be composed of a plurality of different masks mounted so as to enable a selected one of the masks to be installed in the optical path of the collected light propagating towards the detector. The filter may be a programmable LCD or a micro electromechanical structure (MEMS).
The light collecting optics is designed so as to form an angular image of the illuminated region. The term xe2x80x9cangular imagexe2x80x9d signifies such an image of an object, wherein each point of the image corresponds to an angle formed by light coming from a corresponding point of the object. In other words, the collected light ensuing from the light collecting optics is representative of the angular image of the illuminated region formed by the light scattered from the illuminated region and propagated with the certain constant solid angle. The collected light representative of the angular image of the illuminated region is transmitted to the detector.
The light collecting optics comprises first and second optics. The first optics defines the maximum collection angle and is capable of forming a real image of the illuminated region, while the second optics is capable of forming from this real image an angular image of the illuminated region. To ensure that the real image is formed from light components coming from the illuminated region only, the light collecting optics also comprises a slit with a shape and dimensions substantially identical to those of the real image which is mounted substantially at the expected location of this image. This enables any light component coming from a location outside the illuminated region to be prevented from reaching the detector. Hence, the signal-to-noise ratio of detected light is increased even more, considering xe2x80x9cnoisexe2x80x9d as any light component other than that scattered from the illuminated region with the maximum collection angle.
Preferably, the detection unit also comprises an additional optical system accommodated in the optical path of the collected light propagating towards the detector, and is capable of directing said light onto the entire sensing surface of the detector. This enables an undesirable dependence between the detected differences in output signals produced by the detector and typical non-uniformity of its sensitivity distribution to be avoided. Preferably, this additional optical system utilizes a telecentric-imaging mode.
Thus, according to another aspect of the present invention, there is provided an apparatus for optical inspection of an article, comprising an illumination unit generating an incident radiation and Illuminating a predetermined region on the article, and at least one detection unit, wherein said at least one detection comprises:
a light collecting optics that collects light scattered from the illuminated region with a predetermined constant maximum collection angle;
a filter selectively operable in the optical path of collected light for selectively separating therefrom at least one light component propagating with a predetermined solid angle segment of the maximum collection angle; and
an optical system imaging collected light onto the entire sensing surface of a detector.
The detection unit may also comprise a polarizer accommodated so as to be displaceable between its two extreme positions, being, respectively, in and out of the optical path of the collected light propagating towards the detector. The polarizer, when positioned in the optical path, may also be displaceable so as to change the orientation of the plane of its preferred transmission.
Preferably, the detection unit also comprises an image transmission means capable of transmitting the collected light to a desired location of the detector in a manner to avoid any distance-introduced changes of the image. The image transmission means may comprise an imaging fiber bundle formed of a plurality of coherent fibers, or alternatively, may comprise relay optics.
The present invention, due to the above light collecting technique, allows for significantly increasing signal-to-noise ratio in the detected light, xe2x80x9cnoisexe2x80x9d being considered as any light component other than those scattered from the illuminated region with the maximum collection angle. By using more than one detection unit constructed as described above and having light directing optics positioned so as to collect light propagating with different azimuth angles, the defects on the surface of the article are not only detected, but are also successfully classified. The term xe2x80x9cazimuth anglexe2x80x9d signifies an angle between the average direction of an incident beam and that of a returned beam, as seen from the top view. Additionally, due to the provision of the filter, the present invention allows for varying the azimuth and/or elevation angles from their maximum values defined by the light collecting optics. The term xe2x80x9celevation anglexe2x80x9d signifies an angle between the average direction of a scattered light component and the scattering surface, as seen from the side view. This angles"" variation enables differences in the light components representative of the certain angular image of the illuminated region, but propagating with different solid angle segments of the maximum collection angle, to be detected. The provision of the polarizer enables the signal-to-noise ratio to be increased even more, considering the xe2x80x9cpolarized noisexe2x80x9d.
More specifically, the present invention is used for inspecting wafers and is therefore described below with respect to this application.